1. Field of the Invention
The present invention relates to a space navigation device usable as the prime navigation sensor for a completely autonomous spacecraft of satellite navigation system.
2. Prior Art
Systems for determining the angular position between a spacecraft and a source of radiation such as a distant star or other celestial body are known and have been used in a variety of applications. The particular choice of spacecraft navigation system is a function not only of the operation of the spacecraft, but also the type of stabilization system utilized on board the spacecraft. The type of sensors employed can generally be categorized into three broad groupings.
The first grouping comprises analog sensors comprising a pair of photo conductive devices connected as such that a differential output is derived from them. The differential output varies as a function of the projection angle of the vector of the radiation image on the planar surface formed by the photoconductive sensors. These devices, generally known as star trackers, utilize combinations of light baffles and sun shades to provide functions varying as a function of the angular position of the radiant emitting celestial body relative to the detector surface. In order to derive signals indicative of the angular position of the radiant source into two directions, orthogonal to each other, multiple sensors are required. Also, devices of this type require each of the photoconductive sensors to be very closely matched in order to maintain accuracy. The telescopes used in such star trackers are generally large and require extensive thermal and mechanical stability of the photo conductive devices to maintain accuracy and alignment. Hence, the practical application of these devices for making the accurate determinations of angular position which are necessary can be obtained only if the field of view of the detector is relatively narrow, less than 10.degree. of arc. Also, detectors of the photo conductive type generally have an inherently slow response time, and their employment on spin-stabilized spacecraft cannot generally be effectuated. Therefore, the limitations of the use on this type of device as primary navigation sensors have generally been limited with to 3-axis stabilized or gravity gradient stabilized bodies, and in those cases, because of weight restraints, such star tracker devices have been used as a primary sensor, i.e., a lock-on device for a sole star. Also, a further problem inherent with the use of the photo-conductive type of devices of this group is that they cannot usually be employed with lens systems, so that measurements of celestial bodies having relatively low light emission, such as the moon and secondary level stars, are difficult.
A second general type of navigational sensor utilized for space navigation is the reticle time measuring system, and this type of device is applicable only to a spin-stabilized spacecraft. Spin-stabilized bodies generally spin at rates approximating 100 rpm, and a reticle containig a pattern of slits is located in front of a photocell to provide radiation pulses from a celestial body on the photocell in response to the spin of the spacecraft. The time between detection of ajacent radiation pulses is measured to provide the measure of angular position. Obviously, it is necessary to reference the detected time between adjacent pulses to the stable spin period of the spacecraft. The use of analog controllers on the ground, coupled with known orbits, make these particular type of devices relatively simple and inexpensive. However, practical experience has found that the accuracy thereof is limited only to measurements of approximately 1.degree. of arc. Hence, for precision navigation, this type of device is generally unsatisfactory. A modification of the reticle time measuring system, employed for spinning spacecraft, is the digital reticle system. In this type of navigation device, a slit reticle is located in front of a binary of Gray-coded pattern of several photocell detectors. The position of the celestial body image is derived by the binary state of the several detectors in an image plane. For use in spinning vehicles, it is necessary to employ an auxiliary detector to indicate when radiation from the celestial body is in the sensor field of view. Also, to provide measurements of the body in two orthogonal directions, it is necessary to employ a pair of detectors. While digital reticle systems are inherently precise, their resolution is limited as a function of the spectral nature of the radiant energy derived from the body. Working experience with such devices has shown that radiation from the sun subtends an arc of 32 minutes (') which can be detected with the digital type device. If further refinement of the data is required to obtain accuracies, for example, to one arc minute, complex and expensive interpolation using computerized techniques is required. Also, inherent in such devices when utilized on spin-stabilized bodies is that measurements can be derived only once during each spin cycle, and, accordingly, the amount of information which may be derived with such devices is limited.
The patent literature is replete with a variety of attempts of the types mentioned above to achieve a practical and reliable navigation system. For example, a series of patents to Lillestrand, typified by U.S. Pat. No. 3,591,260, and patents to Aroyan et al, typified by U.S. Pat. No. 3,144,555, typify attempts to devise solutions using the reticle system, both in analog and digital contexts. Also, various patents have attempted to devise sextant arrangements which are roughly analogous to the optical star tracker type of devices employed in 2-axis stabilized bodies. As typified by the patent to Carbonara et al, U.S. Pat. No. 2,941,082, a sextant is shown which operates only in a gravitational field by measuring the angle between a heavenly body and a local gravity vector. Also, devices which are hybrids of various techniques are shown. Typical is the patent to Farthing et al, U.S. Pat. No. 3,744,913, which shows the measurement of the center of a radiation emitting celestial body utilizing a detector including four electrodes to determine the image of the body. This type of device is utilized on a spin-stabilized spacecraft primarily as a sun sensor.